This invention relates to a method for recovering flatness of a web material which corrects elongation at side portions of the web material occurring caused by heat treatment for the purpose of the improvement in properties and dimensional stability, and an apparatus therefor.
There are laminated plastic films containing a light-selective membrane, a transparent conductive membrane, etc. which are used in optical or electrical technical fields. The laminated plastic film is heat-treated for the purpose of the improvement in dimensional stability upon heating, of the adjustment of strength, elongation or shrinkage, of drying, polmerization or curing a surface layer coated onto a surface of the film.
Several heat treatment methods of the laminated plastic film are disclosed in Japanese Patent KOKAI 61-74828, 62-127229, etc. In the heat treatment disclosed in Japanese Patent KOKAI 61-74828, one or both side edges of the plastic film are thickened so as to form a space between adjacent film turns upon being wound. In the heat treatment disclosed in Japanese Patent KOKAI 62-127229, the plastic film is heated in an oven up to almost the heat treatment temperature, meanwhile spacers are superposed at least on both side edges of the plastic film. The plastic film heated in the oven is wound up into a roll together with the spacers, and the roll containing spacers which intervene between adjacent film turns is kept at the heat treatment temperature for a predetermined time.
In the above conventional methods, the plastic film is wound to form a roll in a state that either one side edge or both side edges are thickened or accompanied with spacers, and then, heat-treated. Therefore, stress generated in the roll is concentrated on the one side edge or both side edges. Since the roll is heated up to around a glass transition temperature, creep elongation occurs at the side edges greater than the central portion, to degrade flatness. As a result, coating suitability in a subsequent process and transportation suitability are degraded, and loss in product yield increases.